The traditional means of securing flat workpieces to a lapping or polishing carrier has been with some sort of adhesive. One method in common use for example, spin coats a very fine layer of wax solution onto the carrier surface. When the solvent evaporates, a thin film of wax is left behind with a very flat surface. Workpieces or wafers positioned on the wax surface and heated develop sufficient bond to hold them in place without distorting the flat reference plane formed by the wax. Although this method is better than most other adhesive systems in producing flat parts, it shares the drawbacks of difficult clean-up, extreme sensitivity to operator skill, and no tolerance for dirt, dust or contamination.
Many improvements over the wax-mount process are currently in use. Most involve covering the carrier with some kind of material which tends to adhere to the back side of the workpiece. These materials tend to fall into two classifications: those that adhere as a result of surface tack, and those that are spongy and adhere as a result of capillary action or suction. An example of such systems are described in U.S. Pat. Nos. 3,449,870 and 4,132,037. The system described in U.S. Pat. No. 3,449,870 provides some additional protection and security for the workpiece by surrounding it with a retaining fence. The fence is deemed necessary to hold wafers in place that might otherwise slip out from under the carrier.
Waxless mounting systems such as those described in said patents have offered significant improvement in both convenience of operation and clean up. This improvement, however, has been gained at the expense of flatness in the finished parts. As industry, particularly electronics, develops needs for ever flatter parts, the difficulties inherent in all the above systems will make them inadequate.
The primary cause for poor flatness with "waxless" systems is the lack of perfection in the backing or fixturing material. The electronics industry is currently considering specifications for wafer substrates and masks that will require pieces 150 mm in diameter to be flat within one micron. To produce such flatness in a lapped part would normally require the part to be fixtured against a reference surface no less flat. The fixturing materials used for waxless mounting cannot economically or practically be produced with that degree of precision. The lack of precision in these mounting materials can be to some extent compensated for by softness or compressibility in the materials. However, when a plurality of wafers is mounted on a single carrier using a soft or compressible fixturing material, wave effects are set in motion which seriously degrade flatness in the finished parts. Further, if the wafers remain in fixed position, inconsistencies in the mounting material will be reflected through to the detriment of overall flatness.
This invention is directed to a solution of the above problems including providing a means for independently fixturing flat pieces of silicon, glass or other planar materials in order to achieve economically the flattest possible surface during grinding, lapping or polishing operations on said pieces.